Add a GN flag to allow sanitizer builds with full debugging symbols.

build/config/sanitizers/BUILD.gn

Index: build/config/sanitizers/BUILD.gn
diff --git a/build/config/sanitizers/BUILD.gn b/build/config/sanitizers/BUILD.gn
index f034331039269a40e5de0f0ee1be6e992bdd1874..78166cbde89308d996554c48f77a64ab8be9244d 100644
--- a/build/config/sanitizers/BUILD.gn
+++ b/build/config/sanitizers/BUILD.gn
@@ -277,12 +277,15 @@ config("common_sanitizer_flags") {
   cflags_cc = []
 
   # Sanitizers need line table info for stack traces. They don't need type info
-  # or variable info, so we can leave that out to speed up the build.
+  # or variable info, so we can leave that out to speed up the build (unless
+  # it's explicitly asked for by setting |sanitizer_keep_symbols| to true).
   if (using_sanitizer) {
     assert(is_clang, "sanitizers only supported with clang")
-    cflags += [
-      "-gline-tables-only",
+    if (!sanitizer_keep_symbols) {
+      cflags += [ "-gline-tables-only" ]
+    }
 
+    cflags += [
       # Column info in debug data confuses Visual Studio's debugger, so don't
       # use this by default.  However, clusterfuzz needs it for good attribution
       # of reports to CLs, so turn it on there.